CNN Business  — 

Some Instagram users will now be able to message their friends on Messenger, without needing a Facebook account.

The move is the first step in Facebook’s plan to integrate Instagram, WhatsApp and Messenger, announced last year. The effort could effectively cement Facebook’s dominance over the messaging market for years to come, and arguably make it harder to break up the company, which is facing anti-trust scrutiny in Washington. Instagram, WhatsApp and Messenger each boast over 1 billion active monthly users.

The ability for Instagram and Messenger users to message or video chat with each other is currently being tested in select markets and will expand globally in the coming months. The company did not elaborate further at a media briefing, but it said people on Instagram will opt-in to the new experience. It began testing the functionality this summer but is now rolling it out more broadly.

The company declined to provide a timeline for when the same functionality would be available for WhatsApp.

Facebook (FB) first teased plans in March 2019 to intertwine its various services, including WhatsApp, Instagram and Facebook (FB). The idea was that people who only have WhatsApp accounts could message friends on Instagram or Messenger and vice versa, without having to download an app or switch between multiple apps to send messages. Facebook (FB) acquired Instagram in 2012 and WhatsApp in 2014, and spun off Messenger into its own app in 2011.

When asked in a briefing with reporters ahead of Wednesday’s announcement about how difficult it would be to undo the integration if regulators wanted to separate its business, Instagram’s head of product, Vishal Shah, said the effort was a “massive investment from an infrastructure perspective.”

Instagram users will have to opt-in to the ability to chat with people on Messenger.

“The back end systems that we were sitting on top of were not the same. We had to get to a place where they were similar infrastructure. We had to move features and important things over across both Facebook Messenger, as well as on [Instagram] Direct,” Shah said, but he also noted that a lot of the “underlying architecture” between Messenger and Instagram is already shared, such as advertising and commerce.

In addition to being able to chat with each other, Instagram and Messenger will also get some upgrades. Users who opt into the new experience can send “selfie stickers” – a hybrid of Boomerang, selfies and stickers – watch IGTV or Facebook Watch videos together, and turn on a new “Vanish Mode,” in which messages automatically disappear after they’re viewed.

When asked about the possibility that cross-app communication could open the door to harassment, Instagram responded that users can block someone across apps. Additionally, new privacy settings have been added so users can decide who can contact them, and Instagrammers can decide to not receive messages from Facebook users, and vice versa.